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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var a, b;
        (function() {
            alert(a);
            alert(b);
            var a = b = 3;
            alert(a);
            alert(b);
        })();
        alert(a);
        alert(b); //运行结果undefined 多次点击按钮后为3
    </script>
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<body>

</body>

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